1. Field of the Invention
The present invention relates to a gas separation apparatus, more particularly, to a gas separation apparatus using hollow fiber membranes that exhibit selective permeability to gases. The gas separation apparatus of this invention features a compact housing and a removable hollow fiber membrane cartridge positioned therein. The apparatus is particularly useful for dehumidifying compressed air and generating nitrogen enriched air.
2. Description of the Related Art
Fluid separation devices for separating components of a fluid mixture by using hollow fiber membranes having a selective permeability are used in a number of industrial separations including gas separations, dialysis, ultrafiltration, reverse osmosis and the like. The design of the hollow fiber fluid separation devices of these fluid separation applications is taught in U.S. Pat. Nos. 3,722,694; 4,451,369; 4,622,143; 4,623,460; 4,670,145; 4,707,267; 4,781,834; 4,865,736; 4,871,379; 4,881,955; 4,929,259, 5,137,631; 5,211,728; and 5,470,469.
In general, a hollow fiber fluid separation apparatus is comprised of a bundle of hollow fibers constituted within a casing. The bundle is potted with a resinous potting agent at the terminal ends to form tubesheets that provide for a fluid-tight seal between the feed and permeate sides. The resulting bundle is secured removably or permanently in the casing such that the tubesheets divide the casing volume into feed and permeate sections.
The use of various hollow fiber membrane gas separation devices for separating gas mixtures, such as separation of oxygen from nitrogen, recovery of hydrogen from hydrogen-containing gas streams, natural gas sweetening and dehumidification of compressed air is well known in the art. Normally, these separation devices are designed so that the gas mixture can be brought into contact with the hollow fiber membrane therein under a partial pressure differential one or more highly permeable components of the fluid mixture are separated from the less permeable components by permeation through the membrane. The hollow fiber membranes allow the more readily permeable component of the fluid mixture to permeate into the permeate side of the hollow fiber membrane while retaining a substantial portion of the less readily permeable component of the fluid mixture on the nonpermeate side of the hollow fiber membrane. The permeated and the nonpermeated components are removed through or recovered from at least one permeate outlet and at least one nonpermeate outlet, respectively.
In some instances the membrane gas separation devices (assemblies) are designed to provide a purge or a sweep gas on the permeate side of the membrane. The use of a sweep gas on the permeate side of the membrane is beneficial in certain gas separation processes, such as gas dehydration processes, since it decreases the permeate side partial pressure of the more highly permeable component, thus allowing the gas mixture to be more thoroughly stripped of the more readily permeable component. The sweep gas typically flows countercurrently to the direction of the feed/nonpermeate flow. The use of a dry sweep gas can improve the product gas dryness as well as the productivity of the membrane device. A portion of the dry product gas is frequently utilized as the sweep gas generating an internal reflux system. The gas separation assembly that provides for sweep or purge gas introduction generally comprises an annular hollow fiber membrane bundle in an enclosure or a shell having a fluid feed inlet, a nonpermeate outlet, a permeate outlet and a sweep or purge gas inlet. Examples of such membrane assemblies can be found in U.S. Pat. Nos. 3,499,062; 3,735,558; 4,718,921; 5,108,464 and 5,026,479. These fluid separation devices, however, require the use of external plumbing and valves to regulate the flow of the sweep gas to be fed to the sweep gas inlet port. The need to manifold the dry sweep gas external to the gas separation apparatus adds to the size and the complexity of the device. Several attempts have been made to provide an internal sweep gas arrangement and an internal sweep gas flow control. U.S. Pat. Nos. 5,411,662 and 5,525,143 disclose such integral hollow fiber membrane devices.
It is well known in the art to employ hollow fiber membrane modules that consist of multiple internal zones and multiple stages. Such arrangements are employed to provide multiple purity products from a single membrane device, to improve module performance, and to introduce sweep/recycle arrangements. Examples of these module arrangements can be found in U.S. Pat. Nos. 4,220,535; 4,676,808; 5,160,042, and 6,153,097. In U.S. Pat. No. 5,013,437, a hollow fiber membrane fluid separation device adapted for bore side feed that contains multiple concentric zones is described. The device can produce an improved purity nonpermeate product stream. The permeate on the shell side of the device is controlled to maximize concentration gradients along the hollow fibers, thus enhancing the flow of permeate on the shell size of the device and rendering the device more efficient.
In U.S. Pat. Nos. 4,929,259; 4,676,808, and 5,169,530, hollow fiber membrane separation modules are described that contain a discreet zone formed by baffles placed in concentric or radial configurations. The baffles improve fluid contact with hollow fiber membranes. In U.S. Pat. No. 5,158,581, a hollow fiber membrane module is described having segregated active surface regions. In U.S. Pat. No. 5,288,308, a multiple-stage countercurrent hollow fiber membrane module with shell-side feed is described. The module contains multiple separation stages and the means of withdrawing two permeate streams. The apparatus further contains provisions for redirecting a fraction of one permeate stream as a sweep to the second permeate zone through a conduit external to the hollow fiber module. The design of a hollow fiber membrane gas separation apparatus is usually tailored towards specific gas separation processes. For example, a process that requires the use of a fraction of the nonpermeate gas as a sweep on the permeate side of the hollow fiber membrane utilizes a membrane cartridge and a housing enclosure that are substantially different from an apparatus used in a process that does not require the use of the sweep. Furthermore, the cartridge and the housing frequently cannot be used interchangeably for a number of different gas separation applications or, in some instances, even for the same gas separation application that simply requires a different level of product purity. This leads to a proliferation of specialized and frequently unique hollow fiber membrane devices currently employed in the field of gas separations.
The membrane gas separation apparatus is frequently utilized in conjunction with prefiltration equipment, such as coalescing filters, that remove oil and water droplets and a carbon trap that is used to remove heavy hydrocarbon vapors that can be harmful to membrane operation. The prevailing designs of membrane gas separation assemblies frequently make it more difficult to integrate the membrane device with the prefiltration equipment into a single compact gas separation unit. Thus there still exists a need in the art for a hollow fiber membrane gas separation apparatus of a simple design that can be used interchangeably in a number of different gas separation processes, including processes that require the use of a fraction of the product gas as a sweep and that can be integrated with the standard gas filtration equipment into a compact, functional gas separation system.
It is well known in the art that hollow fiber membrane module performance can be improved by increasing module length-to-diameter ratio (high aspect ratio), particularly for high-purity nonpermeate product generation. However, long modules can make it difficult to construct compact systems. It is thus an objective of this invention to provide a compact hollow fiber gas separation module with low length-to-diameter ratio that provides an improved performance for high-purity nonpermeate product generation.
It is another objective of this invention to provide a hollow fiber membrane cartridge that can be interchangeably installed into a standard gas filtration housing and functionally employed in a manner analogous to that of the standard gas filtration equipment.
It is another objective of this invention to provide a hollow fiber membrane gas separation apparatus that can be used in a number of different gas separation applications, including processes that utilize a fraction of the nonpermeate gas as a sweep internal to the device and are capable of generating end products of different purity with no modification to the housing and only an external adjustment to the membrane cartridge.
It is a further objective of this invention to provide a modular hollow fiber membrane gas separation/purification apparatus with feed gas inlet and product gas outlet interfaces that can be easily integrated with the gas prefiltration and post purification equipment into compact gas separation/purification systems.
According to one embodiment of the present invention, the above objectives and other objectives that are apparent to those skilled in the art are achieved by a gas separation apparatus comprising:
(i) a housing body defined by an essentially cylindrical bowl connected in a sealed and removable manner in correspondence with its axial end portion to a lid, wherein said lid having formed therethrough a feed gas inlet port and an outlet nonpermeate gas port and wherein said bowl being provided with a permeate gas exit port placed coaxially to said housing body, and (ii) a substantially cylindrical hollow fiber membrane gas separation cartridge placed coaxially in said housing body and connected in a sealed and removable manner with its first axial end to said feed gas inlet port or nonpermeate gas outlet port in the lid and with its second axial end to said permeate gas outlet port in the bowl said cartridge includes:
(a) an elongated tubular inner core member,
(b) a substantially cylindrical hollow fiber membrane bundle surrounding said inner core member constructed from hollow fiber membranes having permeate and nonpermeate sides, said bundle being characterized as having a substantially countercurrent flow arrangement between the gas flow on said permeate side and the gas flow on said nonpermeate side.
(c) two tubular tubesheets encapsulating both ends of the hollow fiber bundle in a fluid-tight arrangement with one end of the inner core member opening out of one of the tubesheets to permit the flow of gas in and out of said inner core member and wherein at least one of said tubesheets is severed to permit unobstructed flow of gas in and out of the hollow fiber lumens,
(d) a first and second end closure attached to said tubesheets in a fluid-tight arrangement wherein said first end closure is in fluid communication with said waste gas exit port, and wherein said second closure is equipped with a flow control orifice that allows a portion of the nonpermeate gas to be used as a sweep gas on the permeate side of said hollow fibers.
According to another embodiment of the present invention, the above objectives and other objectives that are apparent to those skilled in the art are achieved by gas separation apparatus comprising: (i) a housing body defined by an essentially cylindrical bowl connected in a sealed and removable manner in correspondence with its axial end portion to a lid wherein said lid having formed therethrough a feed gas inlet port and a nonpermeate gas outlet product port, and wherein said bowl being provided with a permeate gas exit port placed coaxially to said housing body, and (ii) a concentric multiple zone hollow fiber membrane cartridge adapted for shell side feed placed coaxially in said housing body and connected in a sealed and removable manner with its first axial end to said feed gas inlet port or said nonpermeate outlet port in the lid and with its second axial end to said permeate gas outlet port in the bowl, said cartridge includes:
(a) an elongated tubular inner core member in communication with said feed gas inlet port or said nonpermeate gas outlet port,
(b) a substantially cylindrical hollow fiber membrane bundle positioned coaxially to and at least partially surrounding said inner core member constructed from hollow fiber membranes having permeate and nonpermeate sides, said bundle being characterized as having a substantially countercurrent flow arrangement between the gas flow on said permeate side and the gas flow on said nonpermeate side,
(c) a first and second tubesheet encapsulating the opposite ends of said hollow fiber bundle in a fluid-tight arrangement with one end of the inner core member opening out of one of the tubesheets to permit the flow of gas in or out of said inner core member, said tubesheets are severed to permit unobstructed flow of gas in and out of the hollow fiber lumens,
(d) at least one impermeable wall member extending longitudinally through said bundle and partitioning said bundle into at least the first and second section with the hollow fiber membranes partitioned between the sections, wherein said impermeable wall member contains a passageway in close proximity to one of said tubesheets,
(e) the first and second end closure attached to said tubesheets in a fluid-tight manner wherein said first closure is in fluid communication with said permeate gas exit port, and wherein said second closure is equipped with a flow control orifice that allows a portion of the nonpermeate gas to be used as a sweep gas on the permeate side of said hollow fibers. In some embodiments the flow control orifice is omitted.